Microstructure and mullitization of aluminosilicate matrix in Nextel 720/aluminosilicate composites prepared by LPCVI at 550°C

Abstract Nextel 720/aluminosilicate composites were prepared by low pressure chemical vapor infiltration (LPCVI) at 550 °C using AlCl 3 –SiCl 4 –CO 2 –H 2 precursor system. The microstructure and mullitization of the aluminosilicate matrix were monitored by SEM, DTA and XRD. The matrix consisted of concentric muti-layers like the annual tree rings and contained some isolated alumina particles and pores. The phase composition of the matrix was composed of an amorphous silica, γ-Al 2 O 3 , Al–Si spinel, an amorphous monophasic mixture (in which Al 3+ and Si 4+ were mixed at the molecular level), and an amorphous diphasic mixture (in which Al 3+ and Si 4+ were mixed at a nanometer scale). The mullitization of the matrix included three stages. Complex mullitization process and inhomogeneous microstructure of the matrix suggested that AlCl 3 and SiCl 4 were not molecularly mixed.